The management of hydrogen sulphide (H2S) plays a major part in the field of gas and oil production. In fact, hydrogen sulphide, which can be present in a molar concentration that can vary greatly depending on the deposits (from a few ppm to several tens of %), is a gas which is not only extremely toxic (fatal at a low concentration) but also corrosive in the presence of water. It is therefore important to treat it, as well as to adopt installations adapted to its presence.
The concentration of hydrogen sulphide can increase dramatically, or even appear, during production, requiring complex and costly adaptations of the production methods. Such an increase or appearance can have several sources, some natural and others artificial.
Thus, examples of possible sources of hydrogen sulphide are:                thermal reduction of sulphates by the hydrocarbons at a high temperature;        bacterial reduction of sulphates;        in the case of injection of hydrochloric acid into the rock during production, reaction of the former with pyrite;        contamination of the considered reservoir of hydrocarbons by a second reservoir of hydrocarbons having a higher H2S content.        
It is important to identify the sources of the hydrogen sulphide for each deposit, in order:                to reduce when possible the hydrogen sulphide content (for example by means of a bactericidal treatment or by interrupting the injection of hydrochloric acid depending on the case); or        to predict the evolution over time of the H2S content, in order to dimension the installations accordingly, preferably to within one-tenth of percent of hydrogen sulphide.        
This identification of the sources of hydrogen sulphide is difficult; in the first place it is based on the isotopic measurement of the sulphur present in the hydrogen sulphide. In fact, depending on the chemical or biochemical processes at the origin of the formation of the hydrogen sulphide, the isotopic fractionation (i.e. the proportion of heavy isotope 34S involved in the various conversion processes) varies. A measurement of the ratio of molar concentration of the heavy isotope 34S to the majority isotope 32S (isotopic ratio) therefore provides direct information on the origin of the hydrogen sulphide, which is a valuable item of information for production strategies.
In order to carry out the measurement of the isotopic ratio of the sulphur, it is usual to oxidize the hydrogen sulphide beforehand. The oxidation-reduction reaction is brought about by bubbling the gas containing the hydrogen sulphide through a solution containing cadmium acetate. The cadmium acetate reacts with the hydrogen sulphide to form cadmium sulphide. The actual isotopic ratio measurement is carried out on the sulphur present in the cadmium sulphide thus obtained.
The measurement of the isotopic ratio of the sulphur is carried out by mass spectrometry. This measurement therefore requires heavy laboratory equipment; it cannot be carried out directly on the site.
Two pre-analysis sampling methods are currently used. These involve:                taking a sample of gas containing hydrogen sulphide on site in a pressurized bottle, and sending the pressurized bottle to the laboratory where the isotopic analysis will take place (the intermediate reaction with the cadmium acetate solution therefore takes place in the laboratory); or        taking a sample of gas containing hydrogen sulphide, carrying out the reaction of hydrogen sulphide with the aqueous solution of cadmium acetate on site, and sending the solution obtained to the laboratory where the isotopic analysis will take place.        
However, these two methods pose considerable problems.
With the first method, the filling of a pressurized bottle is an expensive operation, time-consuming and not very practical to implement under site conditions. Furthermore, H2S is a gas which is toxic for humans, and the transport, generally by air, of samples of compressed gas containing H2S can be a very prolonged operation.
Finally, the quantity of H2S present in the pressurized bottle may not be sufficient to carry out the isotopic measurement.
The second method involves carrying out the oxidation-reduction reaction between the hydrogen sulphide and the cadmium acetate directly on site, and sending an aqueous solution containing a cadmium sulphide precipitate to the analysis laboratory. Whilst this solution is satisfactory in terms of transport, it is difficult to implement. In fact, carrying out oxidation-reduction reactions on site requires fragile laboratory equipment, not very compatible with the site, and personnel qualified to use it. Obtaining reliable and reproducible analysis results requires strict observance of the operating procedure, which is not always possible under site conditions.
Furthermore, for reasons of on-site safety, it is preferable to minimize handling operations under site conditions.
Moreover, the application GB 2344365 describes a device suitable for sampling a determined quantity of fluid of hydrocarbons in situ in a reservoir. A component of the device comprises a material capable of reacting with hydrogen sulphide. The material can be for example a metal, a metal oxide, or an organic compound. However, the system described is mainly intended for measuring the concentration of hydrogen sulphide directly in the fluid of the deposit. It is moreover extremely heavy, complex and expensive to implement due to the difficulty of taking a sample in situ.
Thus, there is a real need to develop a system allowing the isotopic measurement of the hydrogen sulphide which is robust, simple to use under operating site conditions, safe and inexpensive.